1. Field of the Invention
The present invention relates to an oxygen absorbing composition having an oxygen absorbing ability in a low-humidity environment, and to an oxygen absorbing resin composition including such a composition and thermoplastic resin. More particularly, the present invention relates to an oxygen absorbing composition including iron powder and iodine, or an oxygen absorbing composition including iron powder, iodine and metal iodide, and also to an oxygen absorbing resin composition including the oxygen absorbing composition above and thermoplastic resin.
Furthermore, the present invention relates to a product preserving method using the aforementioned oxygen absorbing composition or oxygen absorbing resin composition, especially to a method of preserving medicines or foods etc. which are in a dry state and disfavoring moisture.
2. Description of Related Art
Conventionally, oxygen absorbers utilizing the oxidizing reaction of reduced metals have been widely known. In addition to reduced metal, these oxygen absorbers include metal halide as a catalyst for accelerating oxidizing reaction.
Oxygen absorbers of this type are placed in containers together with food or other products so as to maintain the freshness of such products. In other cases they are adhered to the inside of the containers for the purpose of preserving food or other products.
A typical oxygen absorber of this type is disclosed, for example, in the Japanese Patent (Kokoku) Publication No. SHO 56-33980. This oxygen absorber includes metal powder such as iron powder, and metal halide. More specifically, the oxygen absorber has a structure of metal powder coated with metal halide. An oxygen absorber of this type requires supply of water from the environment upon absorption of the oxygen, and therefore, is called a xe2x80x9cmoisture-dependent oxygen absorber.xe2x80x9d Such an oxygen absorber is used for high water-content foods which permit use of water vaporizing from the preserved product.
On the other hand, in order to preserve dried foods etc. having a low water content (in other words, having low water activity), a typical oxygen absorber includes a water provider for supplying water needed for the oxygen absorbing reaction. An oxygen absorber of this type is known as a xe2x80x9cself-reacting oxygen absorberxe2x80x9d because it is capable of absorbing oxygen without depending on the water vaporing from the packaged substance. Such an oxygen absorber is disclosed in the Japanese Patent (Kokoku) Publication No. SHO 57-31449.
Furthermore, oxygen absorber packets, which are oxygen absorbing resin sheets wrapped in a gas-permeable packaging material and used as a label-, card-, or lid-type, or other types of oxygen absorber for deoxidizing and preserving food or other products are disclosed in the Japanese Patent Laid-Open Publications No. HEI 2-72851, No. HEI 7-137761, No. HEI 7-219430, etc.
However, if a self-reacting oxygen absorber holding water in itself is kneaded into resin, there is a concern that moisture held in the water-holding component might vaporize when heat is applied at the time the oxygen absorber is kneaded and made into a sheet. Furthermore, the vaporized moisture may produce foam inside the resin sheet and cause unevenness of the sheet upon extrusion, whereby formation of the sheet is made difficult.
Moreover, although a self-reacting oxygen absorber is capable of absorbing oxygen regardless of the relative humidity of the environment, moisture included in the oxygen absorber will inevitably vaporize during oxygen absorption and be transferred to the products preserved. Accordingly, there is a problem that the oxygen absorber may not be used if the products preserved are medicines and foods etc. which are in a dry state and disfavoring moisture.
Furthermore, in the case of a moisture-free oxygen absorber, it is possible to include moisture in the aforementioned oxygen absorber after It is kneaded with resin and made into a sheet. However, such a treatment requires a separate step in addition to the steps of manufacturing the oxygen absorber, and is therefore disadvantageous in the industrial manufacture. Furthermore, if an oxygen absorber provided with such a treatment is used for dried products, it is inevitable that moisture held in the oxygen absorber would vaporize and be transferred to the preserved product. Moreover, in the case of some preserved products, transfer of moisture may cause deterioration of taste (dampening), change in quality (powder formed into lumps), chemical change (hydrolysis) and other problems. Furthermore, there is a problem that moisture will vaporize from the oxygen absorber in the course of long-term preservation and the oxygen absorbing performance is reduced, whereby absorption of the oxygen intruding from outside of the container is prevented, the oxygen concentration inside the container is prevented, and the quality of the product preserved is consequently deteriorated.
Furthermore, in the case of an oxygen absorbing multilayered body comprising an oxygen absorbing resin composition as a middle layer, a barrier layer as an outer layer and a seal layer as an inner layer, it is quite difficult to keep moisture in the oxygen absorber layer which is provided as the middle layer.
Accordingly, application of conventional oxygen absorbing resin compositions has been substantially limited to high water-content products, while application to low water-content products has been difficult.
The inventors of the present invention discovered that a new oxygen absorber comprising either iron powder/iodine or iron powder/iodine/metal iodide is capable of rapid oxygen absorption without any water provider even in a low-humidity environment. Free from a concern that moisture may be absorbed by the products preserved, this new oxygen absorber is most suitable for the deoxidization and preservation of food in a dry state. The aforementioned oxygen absorber, which includes iodine or iodine/metal iodide, functions as an accelerator of the oxygen absorption reaction by iron.
The inventors of the present invention further discovered that especially iron powder coated with iodine or metal iodide salt may be favorably blended into resin, made into a sheet and thereafter drawn, and that such an oxygen absorbing resin composition is also capable of absorbing oxygen in a low-humidity environment.
As an oxygen absorber not requiring any water provider, the present invention permits preservation of products of a wider water-activity range. Especially, products having a water activity of 0.1 to 0.7, disfavoring moisture and requiring a dry environment of low humidity upon preservation, for example, dietary foods or powder- or granular-type medicines etc. may be preserved for a long time in a deoxidized state without any moisture being absorbed, whereby deterioration due to oxygen is prevented and the favorable quality of the products may be maintained.
Thus, the present invention provides an oxygen absorbing composition and an oxygen absorbing resin composition employing such oxygen absorbing composition, which demonstrate a sufficient deoxidizing performance even in a low-humidity environment.
Furthermore, the present invention provides a preserving method using an oxygen absorbing composition and an oxygen absorbing resin composition employing such oxygen absorbing composition.
More specifically, the present invention provides an oxygen absorbing composition which comprises iron powder and iodine.
Moreover, the present invention provides an oxygen absorbing composition comprising iron powder, iodine, and metal iodide.
The aforementioned metal iodide may be metal iodide of alkaline metal or metal iodide of alkaline earth metal.
Furthermore, the aforementioned metal iodide of alkaline metal or alkaline earth metal may be sodium iodide, potassium iodide, or calcium iodide.
Furthermore, the sum of the weights of iodine and metal iodide is 0.01 to 20 parts by weight per iron powder 100 parts by weight, and the weight ratio of iodine to metal iodide may be within a range of 0.1-5.
Moreover, the water content in the aforementioned oxygen absorbing composition may be 1% by weight or less.
The surface of the iron powder may be coated with a mixture of the iodine and metal iodide mentioned above.
Moreover, the oxygen absorbing resin composition according to the present invention is capable of absorbing oxygen in a low-humidity environment, and is prepared by uniformly dispersing in thermoplastic resin an oxygen absorbing composition including metal iodide salt and iron powder. The obtained oxygen absorbing resin composition may be processed as a film or a sheet having a single- or multiple-layer construction. Otherwise, the sheet may be further drawn, whereby a porous oxygen absorbing sheet having an improved oxygen absorbing performance is produced.
A further object of the present invention is to provide a method of preserving low water-content products, wherein the aforementioned oxygen absorbing resin composition is wrapped with a gas-permeable material and formed either into a bag-type oxygen absorber, or a label-type oxygen absorber comprising superposed layers of a cover sheet and an adhesive layer for adhesion, and wherein the obtained oxygen absorber is kept in a bag made of a barrier film together with a low water-content product which is to be preserved, whereby preservation of products in a low-humidity environment is made possible.
A yet further object of the present invention is to provide a method of preserving low water-content products, wherein a bag-type oxygen absorber made by wrapping the aforementioned oxygen absorbing resin composition with a gas-permeable packaging material, or a label-type oxygen absorber made by superposing a cover sheet and an adhesive for adhesion is kept in a bag made of a gas-barrier film together with a product to be preserved, whereby preservation of products in a low-humidity environment is made possible.
Specifically, the present invention provides an oxygen absorbing composition which comprises iron powder and iodine, as well as an oxygen absorbing resin composition which comprises thermoplastic resin.
Moreover, metal iodide may be further added to the aforementioned oxygen absorbing composition.
Such an oxygen absorbing composition may be composed of iron powder 100 parts by weight, metal iodide 0.01 to 20 parts by weight, and iodine 0.01 to 20 parts by weight.
Furthermore, the oxygen absorbing composition above may be composed of iron powder coated with the aforementioned iodine and metal iodide.
The metal iodide above may be metal iodide of alkaline metals or alkaline earth metals.
The weight of iodide ion (Ixe2x88x92) included in the aforementioned oxygen absorbing composition may be larger than xc2xd weight of iodine (I2).
Moreover, the water content of the oxygen absorbing composition above may be 1% by weight or less.
Furthermore, the present invention provides an oxygen absorber packet which is made by filling the aforementioned oxygen absorbing composition in a small bag made of a gas-permeable material.
Furthermore, the present invention is also capable of providing an oxygen absorbing sheet having a thickness of 20 xcexcm to 5 mm, which includes the aforementioned oxygen absorbing resin composition.
Furthermore, the present invention is also capable of providing a porous oxygen absorbing sheet which is made by drawing the aforementioned oxygen absorbing sheet 1.5 to 12 times the original size at least in one axial direction.
Furthermore, the present invention provides a bag-type oxygen absorber which is made by wrapping the aforementioned oxygen absorbing sheet with a gas-permeable material.
Furthermore, the present invention provides a label-type oxygen absorber made by laminating a cover sheet, the aforementioned oxygen absorbing sheet, and an adhesive layer for adhesion.
Furthermore, the present invention provides an oxygen absorbing multilayered body prepared by laminating oxygen permeating layers including an oxygen permeating thermoplastic resin, the aforementioned oxygen absorbing sheet, and a gas-barrier layer made of a gas-barrier material.
Furthermore, the present invention provides a method of preserving low water-content products, wherein a preserved product having a water activity of 0.1 to 0.7 is placed and sealed in a gas-barrier container together with the aforementioned oxygen absorber packet.
Furthermore, the present invention provides a method of preserving low water-content products, wherein a preserved product having a water activity of 0.1 to 0.7 is sealed in a gas-barrier container together with the aforementioned oxygen absorbing sheet.
Furthermore, the present invention provides a method of preserving low water-content products, wherein a preserved product having a water activity of 0.1 to 0.7 is sealed into a container, at least a part of which is made of the aforementioned oxygen absorbing multilayered body.
Furthermore, the aforementioned preserved product preferably has a water activity of 0.2 to 0.5.
Furthermore, the present invention provides a package prepared by placing and sealing a preserved product in a gas-barrier container together with the aforementioned oxygen absorbing sheet.
Furthermore, the present invention provides a package prepared by placing and sealing a preserved product in a container, at least a part of which is made of the aforementioned oxygen absorbing multilayered body.
The oxygen absorbing composition according to the present invention includes two components, i.e., iron powder/iodine, or three components, i.e., iron powder/iodine/metal iodide. This oxygen absorbing composition demonstrates a considerably high oxygen absorbing performance in a low-humidity environment as compared with the publicly-known oxygen absorbing composition comprising two components, i.e., iron powder/metal halide.
Iron powder is the main agent of the oxygen absorbing composition, and performs oxygen absorption upon its reaction with the oxygen held in the environment. Iodine, or iodine and metal iodide serve as a catalyst for accelerating the oxidation reaction.
Other components, for example, metal halides may be added to the two-component oxygen absorber according to the present invention, which includes iron powder/iodine. Among metal halides, metal iodides have the strongest effect in accelerating a catalytic action. The three-component oxygen absorber according to the present invention is thus produced.
Regarding the iron powder used in the present invention, any iron powder may be used without specific limitation to its fineness as long as an oxygen absorbing reaction takes place. Accordingly, iron powder having a partially oxidized surface or including other metals may be used. For example, reduced iron powder, electrolytic iron powder, atomized iron powder etc. are preferably used.
Furthermore, crushed or ground products of cast iron etc. are used.
For the favorable contact of iron powder with oxygen, it is preferable to make the particle size of the iron powder small, normally to have a maximum particle diameter of 10 mesh (about 1.7 mm) or less, preferably 50 mesh (about 0.3 mm) or less.
Iron powder having an excessively small particle diameter may generate fire or cause other problems when handled, and is also expensive. Therefore, it is preferable to use iron powder having an average particle diameter of 10 to 500 xcexcm.
The specific surface area of iron powder is preferably 500 cm2/g or more.
According to the present invention, the two-component oxygen absorbing composition comprising iron powder/iodine is, for example, a powdery mixture of iron powder and iodine, which is filled in a small bag made of a gas-permeable material, produced as an oxygen absorbing packet and used for deoxidization and preservation purposes.
The weight of iodine per iron powder 100 parts by weight is preferably 0.01 to 20 parts by weight, more preferably 0.1 to 10 parts by weight. The particle size of iodine is normally less than a maximum particle diameter of 10 mesh, especially preferable if 50 mesh or less.
According to the present invention, the three-component oxygen absorbing composition comprising iron powder/iodine/metal iodide is, for example, a powdery mixture of iron, iodine, and metal iodide, wherein a mixture of iodine and metal iodide preferably adheres to the surface of the iron powder for efficient catalytic effect of iodine and metal iodide. The three-component oxygen absorbing composition, wherein a mixture of iodine and metal iodide adheres to the surface of the iron powder, prevents dispersion of iodide into the air, and demonstrates a favorable oxygen absorbing performance. xe2x80x9cTwo componentsxe2x80x9d or xe2x80x9cthree componentsxe2x80x9d mentioned here are minimum components, and the addition of other components is not considered.
According to the present invention, a catalyst for causing an oxygen absorbing reaction of iron includes iodine and iodide. Electrolytic metal iodide can be used as the metal iodide. Specifically, it is possible to use iodide of alkaline metals, alkaline earth metals, or iodide of transition metals such as copper, zinc, aluminum, tin, iron, cobalt, nickel, cadmium, etc.
Iodides of various metals may be specified as the metal iodides above. However, in terms of safety and catalytic performance, iodides of alkaline metals or alkaline earth metals are preferable, especially sodium iodide, potassium iodide, and calcium iodide. Furthermore, more than two metal iodides may be mixed and used.
In respect to the catalyst for the oxygen absorbing composition according to the present invention, intervention of other halides is not excluded unless the oxygen absorbing reaction is prevented. According to the preferred embodiment, the total content of iodine and iodide in the catalyst is 90% by weight, preferably 95% or more.
Although the reaction accelerator according to the present invention may be mixed with iron powder, it is preferable that the accelerator covers the iron powder. The total weight of iodine and metal iodide in iron powder 100 parts by weight is preferably 0.01 to 20 parts by weight, more preferably 0.1 to 10 parts by weight, and most preferably 0.5 to 6 parts by weight. If the total weight of iodine and metal iodide is under this range, the oxygen absorbing reaction is reduced. On the other hand, if the total weight exceeds this range, the oxygen absorber may absorb excessive moisture, elute and cause a problem.
Concerning the weight ratio of iodine to metal iodide within the oxygen absorber, the weight of iodide ion (Ixe2x88x92) is to be larger than xc2xd weight of iodine (I2). It is preferable that the weight ratio exceeds xc2xd because iodine and metal iodide will then form a complex salt which will enhance the catalytic action for oxygen absorption and contribute to prevent vaporization of iodine into the air.
If the active component for reaction adheres to the surface of the iron powder, and iron powder, iodine and iodide are not separated when blended into the oxygen absorber resin, and sufficient catalytic reaction is expected, the total weight of iodine and iodide may be 0.1 to 10 parts by weight, preferably 0.5 to 6 parts by weight. Furthermore, if the weight ratio of iodine to metallic iodine salt is 1:0.65 to 10, a more preferable oxygen absorbing resin composition is produced.
An example of a method of manufacturing a three-component oxygen absorbing composition is explained below. Foremost, iodine is dissolved in a concentrated aqueous solution of metal iodide, whereby an aqueous solution of iodine and metal iodide is prepared. Thereafter, the obtained solution is sprayed onto and mixed with iron powder as the iron powder is stirred. After drying the iron powder and removing water from the powder, obtained is a granular-type oxygen absorbing composition wherein the surface of the iron powder is coated with a mixture of iodine and metal iodide. The obtained oxygen absorbing composition has a high fluidity and can be easily handled.
Iodine and metal iodide together form a complex salt, which increases the catalytic action for oxygen absorption and contributes to preventing dispersion of iodine into the air.
The water content of the oxygen absorbing composition including two or three components is preferably 1% by weight or less, more preferably 0.2% by weight or less. A water content exceeding this range not only reduces the fluidity of the oxygen absorbing composition powder and makes the preparation of the agent difficult, but also causes a problem of moisture being transferred from the agent to the product preserved.
For preventing malodor, restraining dust, and preventing blots, additives such as siliceous powder, pearlite, diatomaceous earth, aluminum hydroxide, alumina, activated carbon, water-absorbing polymer, etc. may be added to the oxygen absorbing composition as required.
The oxygen absorbing composition including either two or three components is filled in a small bag made of a gas-permeable material and formed into an oxygen absorber packet so as to be used in such a shape for preservation purposes.
As a gas-permeable material, packaging materials having an oxygen permeability of at least 100 cm3/m2/24 hr/atm (2, 50% RH), for example, a plastic film, non-woven fabric, paper etc., or a multilayered film including these materials are used.
Furthermore, the oxygen absorbing composition can be kneaded into a resin, made into a film-type oxygen absorber and used for preservation purposes.
If the oxygen absorbing composition (may also be referred to as the xe2x80x9coxygen absorberxe2x80x9d hereafter) is kneaded into a resin, it is preferable to add oxides of alkaline earth metals, water absorbing agents such as diatomaceous earth, alumina, pulp, water absorbing high polymer, malodor absorbing agents such as activated carbon, molecular sheave, or color pigments such as titanium oxide, iron oxide, carbon black, etc.
Although there is no specific limitation to the thermoplastic resin for blending the oxygen absorber, materials such as polyethylene, polypropylene, various ethylene copolymers, modified polyolefine, elastomers, etc. are preferably used individually or blended.
The oxygen absorbing resin composition according to the present invention is prepared by fusing and kneading the aforementioned oxygen absorber with thermoplastic resin. Furthermore, the oxygen absorbing resin composition can be prepared also by placing the aforementioned oxygen absorber between thermoplastic-resin layers which are softened by heat. The blending ratio of the oxygen absorber to the thermoplastic resin is preferably oxygen absorber 25 to 85 parts by weight in respect to thermoplastic resin 15 to 75 parts by weight, and more preferably, oxygen absorber 30 to 70 parts by weight in respect to thermoplastic resin 30 to 70 parts by weight.
While mixing and kneading the thermoplastic resin and oxygen absorber, the water content within the oxygen absorber is preferably 1% by weight or less. Further water need not be held for the exertion of the oxygen absorbing performance. By performing the procedure above, it is possible to prevent moisture from remaining inside the sheet and unfavorable foams from being generated in the sheet.
Since the oxygen absorbing resin composition according to the present invention is made suitable for preserving low water-content products, it is preferable that the oxygen absorbing composition does not include any water-holding component in itself. The oxygen absorbing composition preferably has a water content of 1% by weight or less, more preferably 0.6% by weight or less because water content in this range allows favorable heat molding without causing any troubles such as foams being generated at the time of thermoforming processing, and a sufficient oxygen absorbing reaction is obtained in this state.
Regardless of its shape, an oxygen absorber comprising the oxygen absorbing resin composition according to the present invention permits rapid deoxidization and long-term preservation of a package holding a product having a water activity of 0.1 or more.
The thickness of the oxygen absorbing sheet is preferably 50 xcexcm to 3 mm, and is to be suitably selected after considering the required oxygen absorption performance, processability, adhesion, filling property, etc.
If the resin composition comprising thermoplastic resin and an oxygen absorber is to be formed into a sheet for use, the thickness of the sheet will vary depending on the manner and the purpose of use. However, thickness under 50 xcexcm may cause problems such as an oxygen absorbing sheet having a very large area required for obtaining a desired oxygen absorbing ability, or iron powder required to be made particularly fine. On the other hand, if the thickness exceeds 5 mm, uniform drawing may not be conducted due to difficulty in keeping a uniform drawing temperature at the time of the drawing processing performed after the sheet processing, or the drawing stress may be so large that processing on a normal machine is made difficult.
Furthermore, by forming the oxygen absorbing resin composition into a sheet and implementing a uniaxial or biaxial drawing, obtained is a porous oxygen absorbing sheet having a number of small voids (microvoids) produced in the thermoplastic resin sheet. The oxygen absorber comprising iodine, metal iodide salt and iron powder uniformly dispersed in the aforementioned sheet, contacts the outside air through the microvoids, and thereby allows effective absorption of the oxygen held in the space inside the packet.
In this case, the drawing rate of the sheet is preferably 1.5 to 12 times the original size. This is because, if the drawing rate of the sheet is less than 1.5 times the original size, not many microvoids will be produced, and the uniformly-dispersed oxygen absorber will not have enough contact with the air and may not show an outstanding oxygen absorbing ability. Furthermore, if the drawing rate of the sheet is more than 12 times the original size, the obtained oxygen absorbing resin composition may not be put to practical use because the film intensity will be considerably reduced in the drawing direction and the oxygen absorber will easily break upon application of a small force. Regarding the temperature for drawing the sheet, any temperature that allows processing may be suitably set. However, microvoids are easily produced if drawing is implemented at a temperature 3 to 7xc2x0 C. lower than the fusing point of the thermoplastic resin, and preparation of an oxygen absorbing sheet having a high oxygen absorbing ability is made possible.
As a product preserving method, a product having a water activity of 0.1 or more is put into a gas-barrier container together with the aforementioned oxygen absorber packet, and the oxygen inside the container is rapidly deoxidized, whereby the quality of the product is preserved over a long period of time. The oxygen absorber packet according to the present invention is characterized by its favorable oxygen absorbing ability in a low-humidity environment. The oxygen absorber packet is powerful in preserving low water-content products having a water activity of 0.1 to 0.6, especially 0.2 to 0.5.
The shape and the material of the gas-barrier container (may be hereinafter simply referred to as the xe2x80x9ccontainerxe2x80x9d) are not limited, and may be selected from, for example, metal cans, glass jars, plastic containers or bags, etc., as long as it can be sealed and has a substantial gas-barrier property.
For example, multilayered sheets or films, or containers or bags made of sheets, having an oxygen permeability of under 0-100 cc/m2/24 hr/atm (25xc2x0 C., 50% RH), more preferably under 0-50 cc/m2/24 hr/atm (25xc2x0 C., 50% RH) are conveniently used. Examples of such gas-barrier containers include: laminated bodies of polyethylene terephthalete/vapor-deposited aluminum/polyethelene, oriented polypropylene/polyvinylalcohol/polyethylene, polyvinylidene chloride coated (K-coat) oriented nylon/polyethylene, aluminum foil/polyethelene, etc., or coextruded laminates of MXD6 nylon.
The oxygen absorbing sheet according to the present invention is devised in various forms, which is preferably an oxygen absorber packet made by cutting a porous oxygen absorbing sheet into small pieces and wrapping them with a gas-permeable packaging material. Examples of such forms include: a bag-type oxygen absorber which is prepared by forming a small bag out of a gas-permeable packaging material and filling an oxygen absorbing composition into such bag; or a label-type oxygen absorber comprising an air permeable composition packet on one side and an adhesive on the other side, which may be easily and conveniently adhered; or a lid-type oxygen absorber in a form of a packing lid, wherein a gas-permeable packaging material is used for attaching the oxygen absorbing resin composition to the center of one side of the barrier packing lid and the packing lid is placed to contact the opening of a jar.
A food package, comprising a label-type oxygen absorber attached thereto, is sealed in a gas-barrier container together with food or other products to be preserved, so that the oxygen absorber absorbs sufficient oxygen and may be effective in preserving the product.
Oxygen absorbing sheets or porous oxygen absorbing sheets comprising the oxygen absorbing resin composition according to the present invention are laminated and provided as an oxygen absorbing layer between an oxygen permeating layer made of an oxygen permeating thermoplastic resin and a gas-barrier layer made of a gas-barrier material. An oxygen absorbing multilayered body is thus obtained. In other words, one mode of the present invention is an oxygen absorbing laminated body comprising at least three layers, i.e., an oxygen permeating layer including an oxygen permeating thermoplastic resin, an oxygen absorbing layer including the oxygen absorbing resin composition according to the present invention, and a gas-barrier layer including a gas-barrier material.
If the oxygen absorbing resin composition according to the present invention is made as a multilayered film or sheet, a gas-barrier layer must be provided at least on one side of the oxygen absorbing resin layer including the oxygen absorbing composition, while an inner layer must be provided on the other side of the oxygen absorbing resin layer.
In the case where the oxygen absorbing resin is made a packaging container, the oxygen permeating layer is provided so as to prevent preserved products from directly touching the oxygen absorbing layer. The oxygen permeating layer may be used also as a sealant layer.
The resin used for the oxygen permeating layer is suitably selected after considering adhesion with the oxygen absorbing layer. The thickness of the oxygen permeating layer is set within a range of 20 to 180 xcexcm, and the oxygen permeability of the layer is preferably 100 cc/m2/atm/day or more, more preferably 200 cc/m2/atm/day or more. Furthermore, pigments, slipping agents, etc. may be suitably added to the oxygen permeating layer.
As a resin for the oxygen permeating layer, thermoplastic resins are preferably used. Examples of such thermoplastic resins include polyolefines such as low-density polyethylene, low-density linear polyethylene, high-density polyethylene, polypropylene, poly methyl pentene, and acidically modified polyolefines thereof, polystyrenes such as polysterene and modifications thereof, various ethylene copolymers such as etylene-vinyl acetate copolymer, etylene-methyl metacrylate copolymer, etylene-ethyl acrylate copolymer, etylene-acrlylic acid copolymer, etylene-propylene copolymer, and elastomers. Among these resins, polyethylene and polypropylene are preferably used due to their high chemical resistance, heat resistance and sanitary property. These resins are used individually or suitably blended.
Regarding the oxygen absorbing composition used for the oxygen absorbing layer, the oxygen absorbing composition above is to be used. In respect to the thermoplastic resin used for the oxygen absorbing layer, various types of resins used for the oxygen permeating layer are used. Activated carbon, zeolite, titanium oxide, and other additives may be added to the oxygen absorbing layer.
The thickness of the oxygen absorbing sheet forming the oxygen absorbing layer is preferably 20 to 200 xcexcm, and is suitably selected after considering the required oxygen absorbing property, processability, mechanical property, etc.
Although there is no specific limitation to the gas-barrier layer as long as such layer avoids permeation of oxygen and moisture, it is preferable to use the following materials: Resin films comprising aluminum foil and other metallic foils or metals or metal oxides such as aluminum, aluminum oxide, silicon oxide evaporated thereon, or nylons such as MXD6 (poly(metaxylene diamine adipamide)), or non-crystal polyamide, or resins such as ethylene-vinyl alcohol copolymer resins, or polyvinylidene chloride, or films coated with polyvinylidene chlorides, all of which may be drawn or laminated and compounded with other resins as required, to an extent not impairing the gas-barrier property. The oxygen permeability of the gas-barrier layer is preferably under 100 cc/m2/atm/day, more preferably under 50 cc/m2/atm/day.
Outside the gas-barrier layer, it is preferable that a protection layer made of thermoplastic resin is further provided. Examples of the thermoplastic resins used for the protection layer include: polyethylenes such as low-density polyethylene, low-density linear polyethylene, high-density polyethylene, various polypropylenes, nylon 6, nylon 6,6, polyethylene terephthalate and combinations thereof.
As for the method of laminating the oxygen absorbing multilayered body according to the present invention, known art such as extrusion and lamination, dry lamination, coextrusion, blow molding can be used, depending on the structure of the multilateral body, and materials and properties of respective layers.
According to the present invention, arrangement and use of an oxygen absorbing multilayered body on part of or the entire inner surface of the packaging container allows absorption and removal of the oxygen generating from a preserved product including peroxide, which has been sealed inside the container. As a result, change in the quality of the product due to oxygen held in the product including peroxide is prevented, as well as change in the appearance, burst or breakage of the container due to rise of the pressure inside the packaging container. The oxygen absorbing multilayered body may be used on the entire walls of a packaging container. Otherwise, the oxygen absorbing multilayered body may be used partly on the walls of the sealed container, while a gas-barrier material not having an oxygen absorbing property may be used on any other wall portions of the packaging container.
Various types of oxygen absorbers or oxygen absorbing multilayered bodies including the oxygen absorbing resin composition according to the present invention demonstrate an oxygen absorbing property in an environment where a relative humidity (RH) is 10 to 70% or 20 to 70%, especially 20 to 50%. Accordingly, these oxygen absorbers or oxygen absorbing multilayered bodies are powerful in preserving low water-content products which have a low water content and generate little moisture, namely products having a water activity of 0.1 to 0.7, or 0.2 to 0.7, especially 0.2 to 0.5, where conventional self-reacting oxygen absorber could not be applied. In other words, these oxygen absorbers or oxygen absorbing multilayered bodies are preferably used for preserving products having low water activity and requiring dry condition and low humidity upon preservation.
Products having the aforementioned water activity fall under the category of low water-content products. However, the present invention may be particularly applied to the preservation of products having a water activity of 0.1 or more and disfavoring moisture, for example, low water-content products (packaged products) such as dried foods, fried snacks and other kinds of foods, powdered or granulated products, medicines, dietary foods, etc. Examples of such products include: powdered soup stock, powdered drinks, powdered sweets, seasonings, powdered grains, nutritious foods, dietary foods, artificial colorings, artificial tastes, spices, powdered or granulated medicines, powder soap, tooth paste, industrial chemicals and compacts thereof (tablet-type products).
Regarding the resin composition according to the present invention, reference may be made to known prior art for any known component, contained amount or manufacturing method. For example, reference may be made to the Japanese Patent (Kokoku) Publication No. SHO 56-33980 filed by the inventors of the present invention.
Details of the preferred embodiments according to the present invention is explained below.